Wild bees when walking the dog

We came across this wild bees nest while walking our son's dog around local playing fields it was good to see that the council had put the sign up rather than removing the risk that someone might get stung.
The tree is an old oak and as the opening was at its base and the inside big enough to house a colony of bees I wonder at the health of the tree.

Just the 29 degrees here West of London and I thought they said it was going to be warm! ;-)

Walking the dog.